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Cloud-building plants help cool the atmosphere (but only slightly)

29 Apr 2013, 15:45

Roz Pidcock and Freya Roberts

We know that plants absorb carbon dioxide, but are they
moderating earth's temperature another way? A new study suggests
the cooling effect of tiny particles emitted from plants can offset
one per cent of global warming. We asked the authors what this
means for slowing the pace of temperature rise.

What are aerosols?

Aerosols are tiny particles that can have a
cooling effect on the atmosphere. The particles scatter
sunlight directly and stimulate clouds to form, preventing sunlight
reaching earth's surface.

Scientists are confident that aerosols offset a substantial
portion of human-caused warming, but there is still a lot of
uncertainty about how much.

Most aerosols come from
human activity, such as vehicle exhausts and wood burning. A
much smaller fraction are naturally occurring, mainly through
volcanic eruptions.

A new study in
Nature Geoscience explains how plants could be another - albeit
much smaller - source of natural aerosols through the gases they
release.

As co-author Ari Asmi from the University of Helsinki tells us,
these so called biological aerosols bolster the effect of aerosols
from other sources. He says:

"In very simplified terms, the more
aerosol particles you have in air, the greater the cooling effect
in the atmosphere".

Plant feedbacks

The researchers collected data from 11 different sites around
the world, measuring concentrations of aerosol particles and plant
gases together with temperature.

They found that warmer temperatures make plants emit more of
particular gases that once in the atmosphere, stick to other
aerosol particles.

This increases the number of aerosol particles big enough to
trigger a cloud to form, leading to a stronger overall cooling
effect. This is an example of what scientists call a negative
feedback.

This isn't the first time scientists have proposed the plant
aerosol theory. But until now, studies have focussed on single
locations, leaving the global effect less well understood. Lead
author Pauli Paasonen from the University of Helsinki tells us:

"[T]his is the first study to show that
this phenomenon takes place on continental scales."

Global effect

Globally, the authors say the effect is very small - offsetting
just one per cent of human-caused warming. In forested areas of
Finland, Siberia and Canada the cooling effect can be much larger -
up to 30 per cent.

The Blue Mountains in Australia,so called because the
organic compounds given off by plants create a blue haze. Credit:
Amanda Slater.

But Asmi tells us that although the researchers are confident of
the mechanism, there is still some uncertainty over the size of the
effect because cloud formation is a complex process. He says:

"The method I used to calculate the
feedback strength is very much simplified ... I would be confident
that there is a cooling effect, but I would be more hesitant about
the strength of it -- there is still much to study there."

Paasonen explains a bit more, saying the group's estimates
should not be considered as exact but as "order of magnitude
estimations."

Rainforests

The new estimate of the global effect of plant aerosols excludes
rainforests. Asmi tells us one reason for this is lack of data. He
says:

"[We have] only recently ... started to
have the necessary international research infrastructure to
actually do these kinds of measurements ... We simply do not yet
have such comprehensive long time series from rain forests."

Another reason is that temperature variations within rainforests
are already very large, so spotting a pattern with increasing
temperature is tricky. But as Paasonen tells us, he doesn't expect
rainforests to drastically affect the outcome:

"I assume including rain forests would
not change the total effect too much ... But, this is something we
are about to study in [the] future. There might be surprises, of
course."

Aerosols represent one of the biggest uncertainties in climate
models, so the new research could help fill in some of the gaps. As
Paasonen explains:

"[U]nderstanding this mechanism could
help us reduce those uncertainties and make the models better."